Two pairs of isomeric resin glycosides from Argyreia obtusifolia

Argyreia obtusifolia is widely distributed in Guangdong, Guangxi, and Yunnan provinces of China and belongs to the Convolvulaceae family. It has been traditionally used for medicinal purposes, such as stopping bleeding and promoting tissue regeneration. The present study was designed to investigate the chemical composition of A. obtusifolia and predict the binding affinity between its chemical constituents and the 6NJS protein. Four new resin glycosides, named argyreia R-U (1-4), along with known compounds (5-9), were isolated from the aerial parts of the plant. The glycoside acid of compounds 1-4 is operculinic acid C (10). Compounds 1 and 3, as well as compounds 2 and 4 are isomers, with the differences between the two pairs of isomers attributed to variations in one substituent and the site of lactonisation. The structures of these compounds were elucidated using 1D and 2D NMR spectroscopy, HR-TOF-MS, and UV and IR spectra. Molecular docking results indicated that compounds 1-4 and 10 were successfully localised within the binding pocket of the protein complex (PDB ID: 6NJS), predicting moderate affinity for the 6NJS protein.